Method of making an assembly of alternator magnet blocks with engine flywheel

ABSTRACT

To secure magnet blocks for an alternator in a cup-shaped flywheel, the inner surface of the flywheel side wall is coated with epoxy and an annular cage is axially inserted into the well in the flywheel. The cage cooperates with inner flywheel surfaces to define radially inwardly opening pockets, into each of which a block is inserted. A tool is disclosed for forcing the blocks radially outwardly to desired positions in which they are held by a fixture while the epoxy is cured.

United States Patent [191 Harkness et al.

[ METHOD OF MAKING AN ASSEMBLY OF ALTERNATOR MAGNET BLOCKS WITH ENGINEFLYWHEEL Inventors: Joseph R. Harkness, Germantown;

John D. Santi, West Allis; Leo J. Lechtenberg, Elm Grove, all of Wis.

Briggs & Stratton Corporation, Wauwatosa, Wis.

Filed: Sept. 16, 1971 Appl. No.: 181,025

Assignee:

US. Cl 29/598, 29/205 R, 156/293, 156/297, 310/45, 310/153, 310/262 Int.Cl. H02k 15/02 Field of Search 29/598, 205 R; 310/153, 310/156, 262, 45;156/293, 297

References Cited UNITED STATES PATENTS 6/1966 Phelon et al 310/153 X[45] June 25, 1974 3,368,275 2/1968 Eberline et al. 29/598 3,680,8226/1954 Brainard 29/598 X 3,727,302 4/1973 Phelon 29/598 X PrimaryExaminer-Charles W. Lanham Assistant Examiner-Carl E. Hall 1 Claim, 10Drawing Figures manna-M2 3,818,586

SHEEI 1 Bf 7 INVENTORB Jaseph H.Hsrkn2ss John 11 5521217.

A-rroa Y PATENTEDJUNZSISM MEI 2 OF 7 |NVENTOR$ Jose 11 H.Hsrkness J:2111 ll SETLi/Z Lea iLeui-zisnbar' BY Arrow Y PATENTED SHEEI 3 OF 7INVENTORS Juseph R-Harknsss John D. 5521M L a JILeaZ-zZEnbEr'g BYA'T'roR EY PATENTEDJUNZSIBH 1818.58

SHEET 8 0F 7 EPOXY RES FIG.7.

luvzwroazs JusE Z-L R Harkness John 17. 55mm v LED JLEUi ZZ/ETLbEZg BYATTo erY METHOD OF MAKING AN ASSEMBLY OF ALTERNATOR MAGNET BLOCKS WITHENGINE FLYWHEEL This invention relates to a rotor having a body whichcan comprise an engine flywheel and which carries a plurality ofblock-like permanent magnets for rotation with the body to cooperatewith stationary elements of a dynamoelectric machine; and the inventionis more particularly concerned with a method and means for securingpermanent magnet blocks to such a rotor body in a predeterminedrelationship to it and to one another. 7

In many small gasoline engine applications it is desired to have analternator that is powered by the engine to provide a source of currentfor charging a starter battery or for energizing headlights or the like.In such installations the alternator often comprises a ring ofblock-like permanent magnets carried by the engine flywheel forcooperation with a core and windings that are stationary on the enginebody at a location adjacent to the flywheel. The flywheel in such casesis made of cast-iron, so that it not only has sufficient mass for itsflywheel function but is also magnetically permeable to provide fluxpaths between the permanent magnets that it carries. Usually, too, theflywheel body is cup-shaped, with a front end wall and a generallycylindrical side wall that enclose the stationary elements of thealternator, and the permanent magnets are mounted in a ring around theinner surface of its side wall.

Various methods and means have heretofore been proposed for securing theseveral permanent magnet blocks to a flywheel body to provide a rotorassembly of the above described character, but all of these priorexpedients have had the disadvantage of being relatively costly.

For example, U.S. Pat. No. 3,390,291, to Eberline et al., discloses aprocedure in which a multiple-member die or mold is used to form a castring in which the magnet blocks are embedded to hold them in a desiredcircumferentially spaced relationship to one another. The cast ring isinserted into the flywheel body after the latter has been heated toexpand it. Then, after the flywheel body has cooled to shrink into tightengagement with the ring, the inner surface of the ring is machined toexpose magnet shoes associated with the magnets. This procedure,involving a multiplicity of operations and relatively expensive moldingequipment, as well as waste of metal that is machined away, is obviouslyexpensive.

US. Pat. No. 3,265,913, to I. J. Irwin, discloses an assembly procedureinvolving a substantially larger number of parts than that of theEberline et al patent, and'wherein the magnets are held assembled withone another and with the flywheel body by means of an arrangement ofclamping rings, screws and other fasteners. While the expedient of theIrwin patent avoids the need for casting and machining operations, themultiplicity of parts involved and the complexity of the assembly aresuch that the Irwin rotor is in the same cost area as that of Eberlineet al.

By contrast with these prior expedients, it is the general object ofthis invention to provide a rotor comprising a flywheel body or the likeand a plurality of blocklike permanent magnets that are secured to thebody in a ring at circumferentially spaced intervals therearound, whichrotor can be manufactured at substantially lower cost than anyheretofore available.

More specifically, it is an object of this invention to provide a rotorof the character described that is very low in cost by reason of thefact that it incorporates a very minimum number of relatively simpleparts and requires for its manufacture a very small number ofoperations, all of which can be performed very quickly and easily byrelatively unskilled labor and with the use of simple and inexpensivetools and equipment.

Another object of this invention is to provide a method and means forassembling individual block-like permanent magnets directly intoacup-shaped rotor body such as an engine flywheel, to dispose themagnets in the proper relationship to the body in the very act ofassembling them into a ring, thus avoiding the necessity for firstassembling the magnets with one or more other components to provide amagnet ring that must subsequently be installed in the body.

Another object of this invention is to provide very simple andinexpensive means for properly locating magnet blocks in a desiredrelationship to one another and to a rotor body during an easilyperformed assembly operation, and for holding them in that relationshipduring the curing of an adhesive material by which the blocks aredirectly bonded to the rotor body.

It is also an object of this invention to provide a method and means forassembling permanent magnet blocks into an annular or cup-shaped rotorbody, and for securing them therein, whereby such assembly can beaccomplished in operations that are so simple and small in number as tobe well adapted to being performed in whole or in substantial part withautomated equipment.

With these observations and objectives in mind, the manner in which theinvention achieves its purpose will be appreciated from the followingdescription and the accompanying drawings, which exemplify theinvention, it being understood that changes may be made in the specificapparatus disclosed herein without departing from the essentials of theinvention set forth in the appended claims.

The accompanying drawings illustrate two complete examples ofembodiments of the invention constructed according to the best modes sofar devised for the practical application of the principles thereof, andin which:

FIG. 1 is a disassembled perspective view of a rotor embodying theprinciples of this invention;

FIG. 2 is a plan view of the rotor in partially assembled condition;

FIG. 3 is a view, partially in side elevation and partially in axialsection, showing the rotor assembled with a tool that is employed forforcing the magnet blocks to their desired positions;

FIG. 4 is a sectional view taken on the plane of the line 4-4 in FIG. 3;

FIG. 5 is a perspective view of a fixture that is installed in the rotorduring the epoxy curing operation;

FIG. 6 is a plan view of a finished rotor embodying FIG. 9 is afragmentary side view of the magnet cage shown in FIG. 8; and" FIG. 10is a greatly enlarged axial sectional view through a portion of aflywheel rotor incorporating the magnet cage of FIGS. 8 and 9,illustrating in exaggerated form the effects thereon of the heat used incuring the epoxy.

Referring now more particularly to the accompanying drawings, thenumeral 5 designates generally a rotor body member which carries a ringof permanent magnet blocks 6, and which is here shown as a flywheel ofthe type commonly used on single-cylinder gasoline engines. The blocks6, when magnetized, cooperate with the stationary elements of analternator (not shown) which can be mounted on the body of an engineupon which theflywheel is installed. As is conventional, the flywheel orrotor body 5 is generally cupshaped, having an end wall 7 and agenerally cylindrical or annular side wall 8, which walls cooperate todefine an axially shallow rearwardly opening well 9. A coaxial bore 10in the end wall is adapted to receive the crankshaft of an engine (notshown) on which the flywheel is mounted.

As is usual with flywheels for small engines, the rotor body member 5here illustrated has vanes 11 projecting forwardly from the exteriorsurface of its end wall to blow cooling air across the engine on whichit is mounted. As is also conventional, a permanent magnet 12 isembedded in the flywheel body and is exposed at its outer peripheralsurface for cooperation with stationary elements of an ignition magneto(not shown) that is mounted on the engine body outwardly adjacent to theflywheel.

The block-like permanent magnets 6 are secured to the flywheel incircumferentially spaced relation to one another in a ring adjacent tothe inner surface of the side wall 8. The blocks are so magnetized thatadjacent ones have opposite polarity, and the rotor body is preferablymade of cast iron to provide magnetic flux paths between them. Thestationary alternator components with which the magnets 6 cooperate aremounted on the engine body within the embrace of the ring that theblocks define, where said alternator components are covered andprotected by the flywheel.

The magnet blocks 6 are of arcuate shape, to conform to the curvature ofthe body side wall 8, and accordingly each has an arcuately convexradially outer surface 14 that is curved on the same radius as the innersurface of the side wall 8, a concave radially inner surface 15 thatprovides a pole face, flat, slightly archshaped front and rear surfaces16 and 17, respectively, and rectangular flat side surfaces 18. Therectangular side surfaces lie in parallel planes, for reasons which willappear as the description proceeds, and hence lie only approximately onradials to the axis of the finished rotor.

In the finished rotor the magnet blocks are held in place by beingbonded to the inner surface of the side wall 8 with an adhesive bondingagent such as epoxy resin. There is no substantial force upon the blocksthat tends to break their bond to the body since they are magneticallyattracted to it and are urged against it by centrifugal force.

An annular cage 19 of nonmagnetic material, preferably cast or molded inone piece, serves to establish the magnet blocks in their properpositions during the assembly operation, retains them in those positionswhile the bonding agent is curing, and protects them in the finishedrotor. The cage can be molded of a plastic material such as nylon, butbecause the coefficient of thermal expansion of most plastics issubstantially different from that of the cast iron of the rotor'body, itis considered preferable to make the cage as a die casting of zinc orthe like.

For assembly of the rotor body, a coating of epoxy resin is firstapplied to the inner surface of the rotor body side wall, all around thesame, and then the cage is inserted axially into the well in the rotorbody, in a predetermined rotational position. The cage cooperates withinner surface portions of the rotor body to define a plurality ofradially inwardly opening pockets 20, one for each magnet block, and themagnet blocks are inserted radially into these pockets. The pockets areof such size that the magnet blocks fit them loosely enough to bequickly and easily installed by hand. As the blocks are inserted, thecage establishes them in desired positions relative to one another andthe rotor body.

By means of a tool 21 (see FIGS. 3 and 4) that is described hereinafter,all of the blocks are simultaneously forced radially outwardly todisplace so much of the bonding material as is necessary to bring theirradially outer surfaces 14 to within a predetermined distance from theinner surface of the side wall 8, and then they are held in thatposition by means of a fixture 22 while the bonding material is cured.

As shown in FIG. 1, the cage comprises front and rear rings 24 and 25and spacer portions 27 which extend axially between the rings andconnect them. The rear ring 24 flatwise overlies the rear faces 17 ofthe magnet blocks. The front ring 25, which can be relatively narrowradially, overlies the front faces 16 of the blocks and extends axiallyforwardly to hold the blocks axially spaced from the end wall and toseal off the space between them and the end wall. The spacer portions27, which are equal in number to the magnet blocks, space the blocksapart circumferentially. Each of the spacer portions is generallychannel shaped, having a pair of circumferentially spaced legs 28 thatextend lengthwise axially and project radially outwardly from a web 29.

As may be seen by reference to the magnet blocks 6 in FIG. 2, which areshown in the positions they occupy in the finished rotor, the radiallyinner surfaces of the webs 29 of the spacer portions are substantiallyflush with the inner surfaces 15 of the magnet blocks; and the radiallyinner surface of the front ring 25 on the cage, at the portion thereofthat is axially adjacent to the magnet blocks, is likewise flush withsaid magnet surfaces and said web surfaces, as best seen in FIG. 3.Hence the cage cooperates with the magnets to provide a magnet ring inthe rotor that has a relatively smooth inner surface. As a result, thecage affords protection to the magnets during assembly of the rotor ontoan engine, and also protects the magnets from being dislodged by flyingstones and the like that might enter through the space between theengine body and the rear edge of the rotor when the engine is in use.

The adjacent legs 28 of circumferentially adjacent spacer portionsdefine opposing pocket surfaces which are parallel to one another andwhich overlie the parallel side surfaces 18 on a magnet block. By reasonof the parallelism of these surfaces of the blocks and pockets, theblocks can have a relatively simple shape that facilitates theirproduction, and they can be easily inserted radially into the pockets20.

Note that no particular reliance is placed upon an adhesive bond betweenthe cage and the rotor body to hold the cage of the FIG. 1 embodiment ofthe invention against displacement relative to the body. Instead, themagnet blocks are mainly relied upon to hold the cage in place by theirengagement with the pocket defining surfaces of the cage. Since manyplastics do not make a good bond with epoxy resin, the embodimentillustrated in FIG. 1 therefore lends itself well to rotor assemblies inwhich the cage is formed of plastic, in

cases where the coefficient of expansion of the plastic presents noparticular problem.

The modified form of cage 19 illustrated in FIGS. 6 and 7 is especiallysuitable for die casting in zinc or other nonmagnetic metal and thusmakes for a rotor assembly that is satisfactory even though subjected tomoderately varying temperature conditions. The metal cage also has theadvantage of affording more protection to the magnet blocks than doesthe plastic cage. In the FIGS. 6 and 7 embodiment the rotor body 5 isprovided with a circumferential ledge or shoulder 32 that extendsradially inwardly from its side wall and is spaced a distance rearwardlyfrom its end wall proper. The front surfaces 16 of the magnets seat onthis ledge.

Since there is no space between the magnets and the front wall of therotor body, the modified cage 19 comprises only a rear ring 24 thatoverlies the rear faces 17 of the magnet blocks and a plurality ofboxlike spacer portions 27 that are formed integrally with the rear ringand project forwardly from it. Each of the spacer portions is preferablycored to form a forwardly opening well 33 therein that saves both weightand material. The opposing pocket-defining surfaces on circumferentiallyadjacent spacer portions are again parallel to one another, as abovedescribed, rather than lying on true radials, to provide for radialinsertion of the blocks 6 into the pockets that are conjointly definedby the cage and the inner surface of the side wall 8.

In this case, since the cage has no front ring, the epoxy is relied uponto bond the cage 19' to the side wall 8 of the rotor body and preventrearward displacement of the cage relative to it, and for this reasonthe box-like spacer portions 27' are formed with radially outer surfacesthat are curved to mate with the inner surface of the side wall and arechamfered at their front edge portions, as at 34. As the cage isinserted axially into the rotor body, the chamfers 34 wipinglydistribute an even coating of epoxy between the spacer portions of thecage and the inner surface of the body side wall.

The junction of the side wall 8 of the rotor body with its end wall 9should be at a rounded corner, to insure structural strength to therotor body and to avoid problems in machining it. However, the adjacentsurfaces of the magnet blocks meet at a square corner. For this reason,as best seen in FIG. 10, the rearwardly offset ledge 32 upon which theblocks are seated has a surface 85 that is inclined forwardly andradially outwardly, so that the blocks rest on only the radially innerportion of that surface. When set in place, the blocks are of courseheld in their proper orientations by their flatwise engagement againstthe inner surface of the side wall.

FIGS. 8 and 9'illustrate another form of cage 19 that is suitable fordie casting in zinc or other metal, and which is well adapted for usewhere the epoxy resin that secures the cage and the magnet blocks to therotor body is cured at a relatively high temperature. Like thatillustrated in FIGS. 6 and 7, the cage of FIGS. 8 and 9 comprises a rearring 24 that overlies the rear faces of the magnet blocks and aplurality of spacer portions 27" that are formed integrally with therear ring and project forwardly from it between circumferentiallyadjacent magnet blocks.

In this case, however, each of the spacer portions comprises a radiallyinner finger 71 and a radially outer finger 72, each of the fingersbeing in the nature of a circumferentially extending wall portion. Theradially inner finger 71 has axially extending ribs or flutings 73 thatreinforce and stiffen it. The radially outer finger 72, however, isrelatively thin, so that it has a slight degree of radial flexibility.Each radially outer finger has its front edge portion chamfered, as at34, so that epoxy is wipingly distributed evenly across the radiallyouter surface of the finger 72 as the cage is inserted into the rotor.

During curing of the epoxy, the cage tends to expand slightly more thanthe rotor body, owing to the high temperature used in the curingoperation and the fact that zinc has a higher coefficient of expansionthan cast iron. As the assembly cools after the curing operation, thering portion of the magnet cage shrinks away from the adjacent surfaceof the rotor body and tends to break the epoxy bond between the cage andthe rotor body as indicated at 75 in FIG. 10. But because of the abovementioned slight flexibility of the radially outer finger 72, thatfinger can assume a slight S-curvature that allows its front end portionto remain well bonded to the rotor body, as at 76. It will be understoodthat in FIG. 10 the S-curve that is imparted to the finger is greatlyexaggerated, as is the separation in the epoxy at 75.

It will be seen that the circumferential side edges of the front andrear fingers cooperate to define side surfaces of the block receivingpockets 20, as at 76, and have sufficient area to insure that the blockswill be properly located as they are installed in the cage. I

For assembly of any of the above described cages into the rotor body,the latter will normally be oriented with its axis vertical and its endwall 7 lowermost, and it can remain in that orientation through thecuring operation. The blocks 6 can be manually inserted into the pockets20 just to the extent necessary to assure that they will not drop out ofthe pockets. However, the permanent magnets 6 are intended to be in amagnetic circuit with the permeable side wall 8 of the rotor body, andtherefore in the finished rotor the distance between the inner surfaceof that side wall and the radially outer surface 14 of each magnet blockshould be as small as possible consistent with the presence of a thinbonding layer of epoxy between them. Hence when all of the blocks havebeen inserted, the above mentioned tool 21. is placed in the rotor bodycavity and is actuated to force the blocks radially outwardly to theirdesired positions, exerting enough force upon the blocks to squeeze allexcess epoxy out from between them and the rotor body side wall.

The tool 21 comprises, in general, a circular body 36 that has adiameter to be received within the cage annulus with a substantialradial clearance, a plurality of plungers 37 one for each magnet block 6that are slidably guided in the body 36 for radially in and out motion,and a coaxial driver 38 which is slidable in the body 36 for up and downmotion and which comprises an upper stem portion 39 and a lowerfrustoconical cam portion 40 that slidingly engages the inner ends ofthe plungers 37.

The body 36 of the tool can comprise two parts, namely, an upper part 41that provides a boss 42 in which the stem portion of the driver isslidably guided, and a lower part 43 that has a plurality of radialslots 44, in each of which one of the plungers 37 is slidable. The camportion 40 of the driver 38 is received in a counterbore 45 whichextends axially through the lower body part and to which the slots 44open at their radially inner ends, and which also extends partway upinto the upper part of the body, where it communicates with a smallerdiameter bore 46 that continues up through the boss and'slidably guidesthe stem portion of the driver.

Each of the plungers 37 has a slot 48 therethrough, intermediate itsends and elongated lengthwise of the plunger, and has a radiallyoutwardly facing shoulder 49. The plunger is biased radially inwardly bymeans of a coiled compression spring 50 which reacts between theshoulder 49 on the plunger and a pin 51 that is fixed in the tool bodyand extends through the slot 48 in the plunger. The radially inner endsurface 52 of each plunger is inclined to correspond with the taper ofthe downwardly convergent frustoconical cam portion 40 of the driver, sothat downward motion of the driver cams all of the plungers radiallyoutwardly against the bias of their springs 50. A pad 53 of rubber orsimilar resilient material on the radially outer end of each plungerprotects the magnet blocks from damage by the plunger and accommodatesslight differences in the combined radial dimensions of the severalblocks and plungers.

The driver 38 is preferably constrained to axial up and down motion bymeans of a pin 55 that is fixed in the boss portion of the body andextends transversely across the bore 46 through axially elongated slotsin the driver stem portion. The upper end of the driver stem ispreferably provided withan enlarged head 56 suitable for engagement bythe ram of a hydraulic press or the like, by which the tool 21 can beactuated.

F IG. illustrates an inexpensive fixture 57 which can be placed in thecage annulus immediately after the tool 21 is removed therefrom, andwhich can remain in the well 9 in the rotor all through the curingoperation to maintain the magnet blocks in the positions in which thetool 21 has established them. The fixture 57 comprises a central disc orplate 58 from which spring fingers 59 project radially outwardly andupwardly. There is of course a spring finger 59 for each of the magnetblocks 6, and each spring finger can be secured to the plate 58 by meansof rivets 60 or the like. The fixture is merely inserted axially, platelowermost, into the well 9 in the flywheel body, with its fingerssubstantially aligned with the magnet blocks, and the spring fingers, inengaging the magnet blocks under resilient bias, hold the fixturecentered in the well.

The flywheel is preferably balanced after the bonding agent has beencured. Since balancing is accomplished by removing cast iron from itsbody, the blocks 6 are preferably installed in unmagnetized conditionsso that they will not attract and collect chips produced during thebalancing operation. The magnet blocks are mag netized in a known mannerafter the flywheel is balanced.

From the foregoing description taken with the accompanying drawings itwill be apparent that this invention provides an extremely inexpensivemethod and means for securing magnet blocks to the inner side wallsurface of a cup-shaped rotor body such as an engine flywheel, and forestablishing the blocks in a predeter- I mined relationship to oneanother and to the rotor body as they are being quickly and easilyassembled with the flywheel.

Those skilled in the art will appreciate that the invention can beembodied in forms other than as herein disclosed for purposes ofillustration.

The invention is defined by the following claims:

1. The method of assembling a rotor that can serve as the flywheel of areciprocating engine and as the moveable element of a dynamoelectricmachine, and which rotor comprises a body having an annular magneticallypermeable wall and a plurality of block-like permanent magnets fixed tothe body in circumferentially spaced relation to one another anddefining a magnet ring adjacent to the radially inner surface of saidwall, which method comprises:

A. forming an annular cage of nonmagnetic material comprising 1. a ringhaving a diameter to be embraced by said annular wall and to overlie oneaxially facing surface of each of the magnets, and

2. a plurality of spacer portions, equal in number to the number ofmagnets, extending in one axial direction from said ring andcircumferentially spaced apart by distances to receive a magnet betweeneach circumferentially adjacent pair of spacer portions;

B. coating the inner surface of said annular wall with a hardenableliquid bonding agent;

C. inserting the cage axially into said annular wall with the cage in apredetermined rotational orientation relative to the body;

D. inserting the magnets radially into the spaces between said spacerportions of the cage and bringing them to positions at which they arewithin a distance from the annular wall which is small enough so thatthere is only a thin, uniform layer of bonding agent between each magnetand said wall; and

E. maintaining the magnets in said positions while hardening the bondingagent.

1. The method of assembling a rotor that can serve as the flywheel of areciprocating engine and as the moveable element of a dynamoelectricmachine, and which rotor comprises a body having an annular magneticallypermeable wall and a plurality of blocklike permanent magnets fixed tothe body in circumferentially spaced relation to one another anddefining a magnet ring adjacent to the radially inner surface of saidwall, which method comprises: A. forming an annular cage of nonmagneticmaterial comprising
 1. a ring having a diameter to be embraced by saidannular wall and to overlie one axially facing surface of each of themagnets, and
 2. a plurality of spacer portions, equal in number to thenumber of magnets, extending in one axial direction from said ring andcircumferentially spaced apart by distances to receive a magnet betweeneach circumferentially adjacent pair of spacer portions; B. coating theinner surface of said annular wall with a hardenable liquid bondingagent; C. inserting the cage axially into said annular wall with thecage in a predetermined rotational orientation relative to the body; D.inserting the magnets radially into the spaces between said spacerportions of the cage and bringing them to positions at which they arewithin a distance from the annular wall which is small enough so thatthere is only a thin, uniform layer of bonding agent between each magnetand said wall; and E. maintaining the magnets in said positions whilehardening the bonding agent.
 2. a plurality of spacer portions, equal innumber to the number of magnets, extending in one axial direction fromsaid ring and circumferentially spaced apart by distances to receive amagnet between each circumferentially adjacent pair of spacer portions;B. coating the inner surface of said annular wall with a hardenableliquid bonding agent; C. inserting the cage axially into said annularwall with the cage in a predetermined rotational orientation relative tothe body; D. inserting the magnets radially into the spaces between saidspacer portions of the cage and bringing them to positions at which theyare within a distance from the annular wall which is small enough sothat there is only a thin, uniform layer of bonding agent between eachmagnet and said wall; and E. maintaining the magnets in said positionswhile hardening the bonding agent.